1. Field of the Invention
This invention relates to a light emitting device using GaN, AlGaN, InGaN or other nitride compound semiconductors, and its manufacturing method.
2. Description of the Related Art
Nitride semiconductor light emitting devices can emit light of short wavelengths of 350 nm or less, oscillation of semiconductor lasers in 400 nm has actually been confirmed. As for their reliability, it has been reported that LEDs have a life as long as ten thousands hours or more. Therefore, nitride semiconductor light emitting devices are hopefully expected to be useful as light sources of optical disc recording systems of next-generation.
For epitaxial growth of nitride compound semiconductors, hexagonal crystals obtained by growth on a sapphire, SiC or spinel substrate are currently considered best. Since sapphire and spine substrates are electrically insulative, both a p-side electrode and an n-side electrode must be formed along the top surface of a nitride compound semiconductor. However, in case of a light emitting device, such as semiconductor laser, which requires injection of a high current density, there occurs a leak current which flows along the surface of the semiconductor. This causes a decrease in amount of current contributing to emission of light, a decrease in light emission efficiency, and a decrease in reliability of the semiconductor laser. By using a current-confining structure, it is possible to suppress such a leak current and increase the current density sufficiently for a semiconductor laser to operate with a low driving current. However, it has been difficult to make a current-confining structure in nitride semiconductor light emitting devices. Therefore, no reliable semiconductor laser, operative with a low operation voltage with no leak current even upon injection of a high current, has been obtained.
Another problem with conventional nitride semiconductor lasers is the noise caused by changes in optical strength during oscillation when a laser used as a data-reading light source in an optical disc recording system. A possible countermeasure is to use a self-pulsation structure, for example. However, it is difficult to invite self-pulsation in conventional nitride semiconductor lasers because the active layer is a thin film. Other possible measures for treating the problem, such as high frequency multiplexing or use of two kinds of lasers, invite structural complexity. There is also a report on incorporating two kinds of laser structures in a single device by locally changing the thickness of the active layer. This approach, however, has revealed the difficulty in controlling the thickness of the active layer. Moreover, although a pickup head for reading and writing is typically composed of two kinds of lasers different in output power, its structure is much complicated.
As discussed, it has been difficult to fabricate inexpensive semiconductor lasers usable as reading and/or writing optical heads for optical disc systems.